This invention relates to flow measurements in narrow channels in general, and in particular to in vivo flow measurements in blood vessels using a method in an interactive system. The invention is applicable when performing coronary flow reserve (CFR) measurements and also when performing fractional flow reserve (FFR) measurements, each taken separately or in combination.
The invention also relates to an interactive graphical user interface and a computer program executing program instructions applicable for performing the method.
In order to measure flow in very narrow channels such as blood vessels, it has been known for a long time to employ a number of different methods, e.g. the so called timed venous collection technique, electromagnetic flow measurements, epicardial ultrasonic flow velocity measurement, the thermodilution technique, and other techniques. For details on these techniques, reference is made to xe2x80x9cMaximal Myocardial Perfusion as a Measure of the Functional Significance of Coronary Artery Diseasexe2x80x9d, by N. H. J. Pijis, (1991), Cip-Gegevens Koninklijke Bibliotheek, den Haag, (ISBN 90-9003818-3).
The present invention concerns improvements in the operation and control of the thermodilution principle, and therefore this principle will be briefly summarized below.
Application of the thermodilution principle in the coronary sinus was introduced by Ganz (Ganz et al, xe2x80x9cMeasurement of coronary sinus blood flow by continuous thermodilution in man, Circulation 44:181-195, 1971). A small catheter is introduced deeply into the coronary sinus and cold saline is delivered at its tip. Theoretically, flow can be calculated from the changes in blood temperature, registered by a thermistor close to the outlet of the coronary sinus. An advantage of this method is that only right heart catheterization is required.
The principle of thermodilution involves injecting a known amount of cooled liquid, e.g. physiological saline in a blood vessel. After injection the temperature is continuously recorded with a temperature sensor attached to the tip of a guide wire that is inserted in the vessel. A temperature change due to the cold liquid passing the measurement site, i.e. the location of the sensor, will be a function of the flow.
There are various methods of evaluating the temperature signal for diagnostic purposes. Either one may attempt to calculate the volume flow, or one may use a relative measure, where the flow in a xe2x80x9crest conditionxe2x80x9d is compared with a xe2x80x9cwork conditionxe2x80x9d, induced by medicaments.
The latter is the simpler way, and may be carried out by measuring the width at half height of the temperature change profile in the two situations indicated, and forming a ratio between these quantities.
Another way of obtaining a ratio would be to measure the transit time from injection and until the cold liquid passes the sensor, in rest condition and in work condition respectively.
The former method, i.e. the utilization of the volume flow parameter as such, requires integration of the temperature profile over time in accordance with the equations given below                               Q          rest                =                              V            /                                          ∫                                  t                  0                                                  t                  1                                            ⁢                                                (                                                            T                                              r                        ,                        m                                                              /                                          T                                              r                        ,                        l                                                                              )                                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                              ∝                      V            /                                          ∫                                  t                  0                                                  t                  1                                            ⁢                                                (                                                            T                                              r                        ,                        0                                                              -                                          T                                              r                        ,                        m                                                                              )                                ⁢                                  ⅆ                  t                                                                                        (        1        )                                          Q          work                =                              V            /                                          ∫                                  t                  0                                                  t                  1                                            ⁢                                                (                                                            T                                              w                        ,                        m                                                              /                                          T                                              w                        ,                        l                                                                              )                                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                              ∝                      V            /                                          ∫                                  t                  0                                                  t                  1                                            ⁢                                                (                                                            T                                              w                        ,                        0                                                              -                                          T                                              w                        ,                        m                                                                              )                                ⁢                                  ⅆ                  t                                                                                        (        2        )            
wherein
V is the volume of injected liquid
Tr,m is the measured temperature at rest condition
Tr,l is the temperature of injected liquid at rest condition
T0 is the temperature of the blood, i.e. 37xc2x0 C.
Tw,m is the measured temperature at work condition
Tw,l is the temperature of injected liquid at work condition
Q is the volume flow
These quantities may than be used directly for assessment of the condition of the coronary vessels and the myocardium of the patient, or they may be ratioed as previously to obtain a CFR, i.e. CFR=Qwork/Qrest.
The latter method, i.e. determination of the transit time requires an accurate time measurement, in view of the relatively small distances in question, about 10 cm or less from injection to measurement site.
E.g. in order to obtain a correct measurement, the time has to be measured with some accuracy. Using a simple stopwatch, which is a common means of timing, is far too inaccurate for obtaining reliable transit times.
The flow F may be obtained as follows, which is a derivation for a similar technique, namely the indicator dilution technique. This is based on a rapidly injected amount of some kind of indicator, the concentration of which is measured.
For this purpose, the function h(t) is introduced which is the fraction of indicator, passing per unit of time at a measurement site at time t. In other words, h(t) is the distribution function of transit times of the indicator particles. If it is assumed that that flow of the indicator is representative for flow of the total fluid (complete mixing), h(t) is also the distribution function of transit times of all fluid particles. Suppose the total volume of fluid is made up of a very large number of volume elements dVi which are defined in such a way that dVi contains all fluid particles present in the system at t=0, with transit times between ti and ti+1. The fraction of fluid particles requiring times between ti and ti+1 to pass the measurement site, is h(ti) xcex94t by definition, and because the rate at which the fluid particles pass at the measurement site, equals F, the rate at which the particles making up dVi pass at the measurement site is Fxc2x7h(ti)xc2x7xcex94t. The total volume of dVi equals the time ti, required for all particles segments in dVi to pass at the measurement site multiplied by the rate at which they leave. In other words:
dVi=tixc2x7Fxc2x7h(ti)xc2x7xcex94t 
and by integration:   V  =      F    ⁢                  ∫        0        ∞            ⁢                        t          ·                      h            ⁡                          (              t              )                                      ⁢                  xe2x80x83                ⁢                  ⅆ          t                    
The integral in the equation above represents the mean transit time Tmn, which is the average time, needed by one particle to travel from an injection site to a measurement site. Therefore:
V=Fxc2x7Tmn 
or:
F=V/Tmn; Tmn=V/F 
which states the fundamental fact that flow equals volume divided by mean transit time.
Although the above derivation was made for the mentioned indicator dilution technique, the result is the same for thermo-dilution since the same distribution function may be employed.
Applicant""s own International Patent Application WO 97/27802 entitled xe2x80x9cCombined flow, pressure and temperature sensorxe2x80x9d, and filed Jan. 30, 1997, discloses a combined pressure, temperature and flow sensor. Therein, the utility of the combined sensor is discussed in terms of application of the hot anemometer technique, and the thermodilution technique.
However, the time measurement is triggered by the cold saline passing a temperature sensor at a point upstream of the measurement point. This requires a special guide catheter provided with a temperature sensor.
U.S. Pat. No. 6,089,103, also assigned to the same applicant as the present application and relates to a method of flow measurements in a narrow channel. A sensor is provided, having an element sensitive to pressure, and an element sensitive to a selected chemical or physical property, e.g. temperature. When a thermo-dilution measurement is to be performed a guide catheter is filled entirely all the way up to the distal opening with cold saline, at a temperature lower than the blood temperature. Then a small bolus amount is injected into the guide catheter, normally at its proximal end. Thereby a corresponding amount will be expelled from the distal opening of the guide catheter and into the blood vessel, and thereby transported towards the measurement point by the flowing blood. When the cold saline passes a temperature sensor on the distal tip of the guide catheter, the temperature sensor registers a temperature gradient and in response thereto a timer in a control unit is initiated. Again, when the bolus of cold saline passes the sensor at the distal tip of the guide wire, a temperature gradient is recorded and the software in a control unit processes the recorded data and outputs a result a value of a flow parameter.
EP-1,125,548 (published after the priority date of the present application) relates to measurement of flow in blood vessel by thermo-dilution and in particular to a method of triggering such measurement in order to improve the measurements. In the EP-application the inventors have realized that a previous problem acknowledged in connection with thermo-dilution can be used to an advantage for triggering purposes. Namely, when a bolus of cold saline is injected into a catheter where a wire carrying the sensor unit and electrical leads for signal transmission is located, the lead resistance will be instantly affected by the cold saline by a change in the resistivity. This is a problem in that the change must be compensated for in order to arrive at a correct output signal. In accordance with the invention in the EP-1,125,548, the resistivity change is recorded as a resistance variation curve, and various parts of the recoded curve, or the entire curve, can be mathematically processed to give as a result a starting point for the determination for a transit time of the injected liquid. The starting point can be determined in different ways. According to one way the onset of the resistivity reduction is registered and the derivative of the curve is calculated, and if the derivative exceeds a preset value, time measurement is triggered. Another way is to use the peak value as a stating point for time measurement. Again the derivative, or preferably the second derivative, is calculated and the change in sign is detected. In this way the accuracy in the time measurement is significantly improved.
In order to take full advantage of the improved accuracy in the time measurement which of course also influences the overall measurement accuracy of CFR and also FFR the physician performing the diagnostic measurements must be able to easily and safely be guided through the measurement procedure, and also have the relevant measurement results presented in a way that gives a clear overview of all aspects of the results.
The object of the present invention is to achieve an improved method in a flow measurement system including an improved interactive user interface.
The above-mentioned object is achieved by a method according to the independent claim.
Preferred embodiments are set forth in the dependent claims.